1. Field of the invention
The present invention refers to a method and a device for sensorless control of a reluctance motor in dependence on motor parameter values measured in the output stage of the motor. By processing in a microprocessor of said values, at predetermined values a commutating signal is initiated for the actual phase of the reluctance motor.
2. Description of the prior art
A reluctance motor can be designed for single-phase or multi-phase operation and has a stator with excitation windings arranged polewise. In a common type reluctance motor the stator comprises four poles while the rotor has two poles. The four poles of the stator are arranged so as to form a cross and the rotor has a design locating its poles in diametrically opposite positions. In this 4/2-pole motor the stator poles are activated in pairs twice per revolution, whereby the pole pair, thus activated, generates a magnetic field forcing the rotor to take the position in which the magnetic resistance, or the reluctance, of the magnetic circuit has its minimum value. For the continued operation of the rotor a commutation is required of the current supply to the stator poles so that the stator pole pair to folio be activated at the same time as the stator pole pair presently supplied is cutoff.
Normally, this commutation is controlled by voltage or current pulses being synchronized with the position of the rotor. In order to achieve this, usually, one or several rotor position sensors are being used which generate control signals in dependence on the angular position of the rotor. The sensors are optical or magnetic type sensors resulting in the need for an extra element to be disposed adjacent to the rotor. In practice, the use of such rotor position sensors has been found to considerably complicate the manufacture and installation of the motor.
Therefore, various types of sensorless commutation control have been suggested. In which some kind of measuring and/or calculation of the Inductance of the motor has been used. Accordingly, it is known to make inductance calculations based upon:
1. the measurement of the frequency and/or the amplitude of an oscillator connected to the winding of the respective stator pole pair (U.S. Pat. No. A4,520,302), or upon
2. the measurement of the derivative of the current in the winding during a voltage pulse applied (SE-B-8604308-0), or upon
3. the calculation of the magnetic flux by dividing the applied voltage by the current measured (SE-B-8604307-2).
In principle, in all of the solutions presented a measurement of the inductance, or the differential inductance, of a phase takes place and the value thus measures is then compared with a limit value for the commutation. The known solutions operate in a bad way or doesn't operate at all when the reluctance motor is operated at high speed or at high load. In the latter case the motor is saturated, i.e. the inductance changes due to the fact that the current changes. Various variants of current compensation have been tested with varying success. However, such current compensation causes the construction of the circuit to become more complicated.